A person with post-traumatic amnesia has a hole where an event should be.

And the human brain will not tolerate a hole.

It is not built for one.

So when you ask a concussed patient to tell you what happened — and everybody asks, the paramedic asks, the doctor asks, the father asks at twenty past one in the morning — the brain supplies material from adjacent memory and presents it with the same confidence as everything else.

It is called confabulation and it is not lying and the patient has no way of telling the difference from the inside.